Known in the art is a tube-and-fin heat exchanger employed as a water-to-air radiator on motor vehicles, tractors and diesel locomotives. This apparatus comprises flat or round tubes intended for the passage of the coolant flow and installed in appropriate broached holes provided in flat plates serving as cooling fins. The coolant tubes may be disposed in parallel or staggered rows. With this construction, plain rectangular ducts are formed between the tubes, said ducts having no turbulence producing means required for intensifying the heat exchange process in the intertubular space.
Said means for intensifying the heat exchange process have to be provided because the water-to-air radiators of various power installations operate under conditions where the radiator heat transfer coefficient K is approximately equal to the air heat transfer coefficient .alpha..sub.1, i.e., K.apprxeq..alpha..sub.1. Therefore, decreasing the volume and mass of a water-to-air radiator necessitates increasing K which is uniquely determined by the value of .alpha..sub.1. As is known, plain ducts give the least values of .alpha..sub.1. Therefore, the known tube-and-fin heat exchanger has a substantial size and mass.
To decrease the size and mass of the water radiators of the known type, the air heat transfer coefficient .alpha..sub.1 has to be increased, which can be accomplished only by setting up turbulence in the air flow through the radiator passages by the agency of various turbulence producing means.
Also known in the art is a tube-and-fin heat exchanger comprising flat tubes intended for the passage of the water being cooled and installed in parallel or staggered rows in a stack of fins. In order to intensify the process of convective heat transfer in the intertubular space, the fins are profiled in the direction of the cooling air flow as a continuous symmetrical wavy line, whilst adjacent fins are installed in the tube bank so that the projections and depressions of said fins are disposed equidistantly with respect to each other. Consequently, between adjacent fins cooling air ducts are formed which have a wavy profile in the direction of the air flow.
The analysis of the results of tests of the water-to-air radiators of the type under consideration shows that such radiators give little thermohydraulic effectiveness inasmuch as the increase of the air heat transfer coefficient .alpha..sub.1 in the aforementioned ducts substantially lags behind the increase in the energy expended in intensifying heat transfer therein, as compared with similar plain ducts. This is attributed to the fact that when air flows in such ducts a vortex system is set up after each turn and therebefore, said system being equal in scale to or commensurable with the height of the projection in the wavy duct, whereas the height of the projection in such ducts is equal to or commensurable with the duct hydraulic diameter. As a result, up to 70-80 percent of the supplementary energy supplied to the cooling air in said wavy ducts is expended in setting up turbulence in the flow core where the gradients of the temperature field and the density of the thermal flow are small, which entails little increase in the density of the thermal flow. Since these large-scale vortex systems possess substantial kinetic energy, they, overcoming viscosity and friction forces, gradually become dissipated and enter the air layer at the walls. As a result, turbulence is set up in said air layer with consequent increase of turbulent conduction and density of the heat flow. Therefore, intensification of heat transfer in the wavy duct is effected mainly by setting up turbulence in the flow layer at the wall, not in the flow core, although the greater part of the supplementary energy supplied to the air flow in the wavy duct is expended in setting up turbulence in the flow cre, not in the layer at the wall. This is the reason for low thermohydraulic effectiveness of the heat transfer surface of said tube-and-fin heat exchanger known in the prior art.
Also known in the prior art is a tube-and-fin heat exchanger comprising a stack of fins spaced apart. The tubes are installed in broached holes provided in the fins. One heat-transfer medium flows through the tubes. Adjacent fins and the walls of adjacent tubes form ducts for the flow of the other heat-transfer medium whose temperature differs from that of the first-mentioned heat-transfer medium. Heat transfer is effected between said media. Each of the fins is made in the form of a continuous symmetrical wavy line. In order to intensify the process of convective heat transfer, the projections and depressions on each fin are located respectively opposite the projections and depressions on the adjacent fins. With this construction, continuous divergent-convergent duct portions are formed in the direction of heat carrier flow, the divergence angle being substantially greater than the critical angle for the initial upsetting of hydrodynamic stability of the laminary structure of the heat carrier flow. This results in setting up three-dimensional twisted vortices in the boundary layer. Eddy viscosity and conduction sharply increase in this layer. The temperature gradient and the density of the thermal flow increase, entailing increase in the coefficient .alpha..sub.1 of heat transfer between the heat carrier and the walls of the divergent-convergent ducts. Energy-consuming vortices are generated in the divergent portions of the ducts under certain conditions of throttling and heat carrier flow. The interaction of the vortices therebetween and with the main flow of the heat carrier causes diffusion of said vortices into the flow core. The total energy of generation and propagation of the vortices exceeds the energy of their dissipation. Therefore, the expenditure of energy on forcing the heat carrier flow increases materially with insignificant increase in the intensification of the heat transfer. This physical characteristic of the heat transfer intensification process inherent in the apparatus under consideration entails substantial decrease in the thermodynamic effectiveness thereof.